Joining compositions are compositions that may be used as sealants or adhesives and that join one surface to another, such as by filling a gap between two surfaces or by adhesively connecting one surface to another. In the case where the joining composition is intended as a sealant, the purpose of the composition is for sealing seams and joints in various structures. Sealant compositions often are used in the building industry to eliminate gaps between building elements to prevent ingress of water, dust, insects, and other unwanted factors. Such sealant compositions may also aid in thermal insulation, water resistance, and noise reduction. Elastomeric sealant compositions are desirable where the materials forming the joint are anticipated to move due to thermal expansion or contraction.
When the joining composition is intended for use as an adhesive, the purpose of the composition is to join one surface to a second surface. Again, a large variety of adhesives are known in the art, and such adhesives are used in many commercial applications including the residential and commercial construction industries. Roof sealants and sealants for use with glass, concrete, and aluminum all are known, for example.
Many commercial sealant and adhesive compositions are made with silicone, polyurethane, polysulfide, silyl-terminated polyether, or acrylic compounds. The choice of sealant compounds generally depends on the intended properties and applications. For instance, silicone sealant compositions are used in a wide variety of building applications due to generally good UV resistance, temperature resistance, movement capability, flexibility, and service life. Most silicon sealant compositions, however, do not accept paint well. Polyurethane sealant compositions, on the other hand, are generally desirable due to toughness and abrasion resistance. Polyurethane sealant compositions can generally be painted, but are often stiffer than silicone sealants. More recently, “MS Hybrid” (silyl-modified hybrid) sealant compositions have been developed in an effort to combine some of the strengths of both silicone and polyurethane sealants.
Sealant compositions generally are formulated with plasticizers, which are compounds that enhance the plasticity of the sealant composition. It is known that some plasticizers commonly used in MS Hybrid systems to increase flexibility or workability are known to migrate out of the system over time. Because plasticizers are joining substances, the migration can result in high dirt pick-up. In particular, many plasticizers used in MS hybrid sealants are phthalate-based. Such plasticizers are generally compatible with the silyl-modified base compound, but it is known that these plasticizers slowly leach from the sealant and can invade neighboring materials. This not only can reduce the flexibility of the sealant compositions and lead to dirt pick-up, but also can potentially undesirably alter adjacent materials and present potential environmental and toxicity concerns. Non-phthalate plasticizers exist, but there remain compatibility concerns. If the plasticizer is not compatible with the other components of the sealant composition, it will eventually migrate to the surface and either be flushed away, migrate into other materials, or adhere to dirt and other debris. In addition, some plasticizers used in MS Hybrid systems act as a food source for mildew and fungus, which can potentially lead to undesirable microbial growth.
Compositions that are useful as sealants are sometimes chemically similar to certain adhesive compositions, although in general sealants and adhesives generally have somewhat different properties. Sealants are used to close gaps between surfaces, and for this reason sealant compositions generally should have high elongation and flexibility and should exhibit low shrinkage after application. Sealants are often used as finishing compositions, and therefore resistance to mildew and dirt pick-up is highly desirable. Adhesives, on the other hand, are compositions that are used to join two surfaces together. Adhesives often have higher strength than sealants, and the resistance to mildew and dirt pick-up is of less importance because the adhesive composition is generally not visible upon curing. Nonetheless, in many cases properties desirable for an adhesive are similar to those desired of a sealant. For example, it is often desirable that an adhesive composition have some plasticity to prevent failure of the adhesive joint upon flexing or thermal expansion and contraction. It is further desirable that the composition be stable over time, such that any plasticizers in the composition are not prone to leaching out.
It is desired to provide a joining composition that is stable and that has sufficient plasticity for use as a sealant composition, in some embodiments, or as an adhesive composition, in other embodiments.